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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods                           9  TCP/IP Internet Message Access Protocol (IMAP/IMAP4)

IMAP Overview, History, Versions and Standards 1 2 3 IMAP Commands, Results and Responses

Session States

The session between an IMAP4 client and server is described in the IMAP standards using a finite state machine (FSM), a descriptive tool that I explain in the TCP operational overview. Again, this is similar to how the operation of POP3 is explained, except that IMAP4 is a bit more complex. Its FSM defines four states instead of three, and where a POP3 session is linear (only going through each state once) in IMAP4 the session is not. However, the state flow is still fairly straight-forward, mostly following a logical sequence from one state to the next. The IMAP FSM is illustrated in Figure 309.

The following are the IMAP states, in the usual sequence in which they occur for a session:

1. Not Authenticated State: The session normally begins in this state after a TCP connection is established, unless the special IMAP preauthentication feature has been used (we’ll get to this feature shortly). The client at this point cannot really do much aside from providing authentication information so it can move to the next state.
   
   
2. Authenticated State: The client has completed authentication, either through an authentication process in the prior state or through preauthentication. The client is now allowed to perform operations on whole mailboxes. The client must select a mailbox before individual message operations are permitted.
   
   
3. Selected State: After a mailbox has been chosen, the client is allowed to access and manipulate individual messages within the mailbox. When the client is done with the current mailbox it can close it and return to the Authenticated state to select a new one to work with, or can log out to end the session.
   
   
4. Logout State: The client may issue a Logout command from any of the other states to request that the IMAP session be ended. The session may also enter this state if the session inactivity timer expires. The server sends a response and the connection is terminated.

Of the four IMAP states, only the first three are “interactive”, meaning states where commands are actively issued by the client and responses provided by the server. Some IMAP commands can be used while the session is in any state, while others are state-specific. The next topic describes IMAP commands and responses in general terms.

IMAP Overview, History, Versions and Standards 1 2 3 IMAP Commands, Results and Responses